1. Field of the Invention
The present invention relates to a two-side film forming method for forming, for example, optical thin films on both faces of a substrate such as a lens, and an apparatus therefor.
2. Related Background Art
The optical thin film, such as antireflection film, is often formed on both sides of a substrate such as a lens, and there has widely been employed a method of forming the film on one side at a time, for example by resistance heating or electron gun heating. However such method requires a long time and involves the operation of inverting the substrate inside the vacuum chamber, and is, therefore, complex in the process itself and in the apparatus. For this reason there has been developed a method of simultaneously forming the films on both sides of the substrate by sputtering, as disclosed in the Japanese Patent Laid-Open Application No. 52-65184. This methods consists of positioning the substrate between at least a pair of targets provided in a vacuum chamber and depositing the particles, released from the surfaces of the targets onto both sides of the substrate, and is based on a fact that the direction of particle generation can be arbitrary, for example inclined or downward in case of sputtering, in contrast to the resistance heating or electron gun heating in which the generated particles are directed vertically upward.
Such simultaneous film forming on both sides of the substrate provides an additional advantage of avoiding thermal strain, since both faces of the substrate can be maintained free from temperature difference even if the substrate is heated for example by ion irradiation.
However, in the conventional method mentioned above, it is difficult to obtain an optical thin film of satisfactory optical characteristics particularly in the case of film forming with a fluoride, such as MgF.sub.2 which is recently attracting particular attention as the material for the optical thin films. In the sputtering method, the material for thin film is once formed as a plate-shaped target and is evaporated by the impact of accelerated particles, however, the quality of the formed film is difficult to control in certain materials. For example, in case of forming an optical thin film containing MgF.sub.2 etc., the sputtering method cannot provide a film of good quality with a low absorbance.
Also cluster ion beam evaporation method has been known for obtaining a thin film excellent in optical characteristics, adhesion and durability such as antiabrasion, without heating of the substrate to a high temperature.
FIG. 1 illustrates an ordinary apparatus for cluster ion beam evaporation, in which a closed crucible 131 is provided in a vacuum chamber 110, maintained at a predetermined reduced pressure by means of a vacuum pump, connected to an exhaust opening 111a. A material for thin film, contained in the closed crucible 131, is heated by a heating bombardment filament 132, and thus generated vapor is emitted, from a nozzle 131a of the closed crucible 131, into the vacuum atmosphere to form a cluster (clustered atoms). Then the clustered vapor (cluster particles) is irradiated with electron shower generated from an ionizing filament 133 to ionize at least a part of the cluster particles, and thus generated ions accelerated by an accelerating electrode 134 to irradiate a substrate W.sub.0.
In such cluster ion beam evaporation, the vapor of the material for forming the thin film is deposited onto the substrate, in the form of a cluster particle, consisting of loose combination of a large number (usually more than 1,000) of atoms.
Such cluster ion beam evaporation has been employed in the film formation on one side of the substrate.